Compression across the board: directory answers, uploads, metrics, traces, the page (#253)

Measured 2026-10-16, uncompressed until now: the directory's answers (the feed list ipx reads,
603 KB of JSON, ~134 ms of a 159 ms Directory request), ipx's snapshot upload (~100 KB each half
hour), both /metrics (4.7 and 0.9 KB every 15 s) and every trace export to Tempo. Already
compressed: ipx-web's answers (zstd, #239), the home page and the directory's public pages
through Cloudflare (brotli, zstd).

- The directory compresses with brotli, zstd or gzip as asked, and unpacks a gzipped request.
- ipx sends its snapshot gzipped; the directory is deployed first.
- Both /metrics answer gzipped, as Prometheus asks.
- opentelemetry-otlp's gzip-http: OTEL_EXPORTER_OTLP_COMPRESSION=gzip compresses the export, set
  for all three in production's compose file; Caddy's exporter reads the same variable.
- web/build.mjs writes .br (quality 11) and .gz (level 9) beside each text file, and Caddy serves
  them in place (file_server precompressed): app.js 30 KB where zstd on the fly gave 37, app.css
  11 where 14. A .br or .gz asked for by name is refused, or /admin.js.br would be the admin
  page's script without the check that keeps it to admins; in a handle of its own, since a bare
  respond runs after the handles.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/ipx/CHANGELOG.md

@@ -22,6 +22,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- With `OTEL_EXPORTER_OTLP_COMPRESSION=gzip`, traces go to the collector compressed; `/metrics` answers gzipped, and a snapshot goes to the directory gzipped (#253).
 - A Directory request's trace shows the call to the directory whole, with the size of its answer, and the steps that match it against your subscriptions (#250).
 - The page's report of how long a list took to show after a click (`list_shown`, with the view and filter) is logged under that click's trace (#246).